This invention generally relates to an improved process for preparing derivatives of mercaptophenols. Further, the invention provides for an improved process for preparing hindered phenol sulfides containing a nitrile, carbonyl, sulphone or ester group from the reaction of mercaptophenols with vinyl compounds containing a conjugated nitrile, carbonyl, sulphone or ester group.
The prior art generally teaches the preparation of mercaptophenol derivatives which are extremely useful as stabilizers for various organic and polymeric materials. A number of mercaptophenol derivatives are known to be useful as antioxidants and in pharmaceutical applications. In particular, Japanese Application No. 47-115575 discloses the preparation of mercaptophenol derivatives containing nitrile or carbonyl groups. European Patent Application No. 82810476.0 discloses mercaptophenols containing ester groups with improved stabilizer performance and U.S. Pat. No. 4,311,637 discloses polyphenolic esters of mercaptophenols useful as antioxidants.
The compounds disclosed by the prior art are generally produced by reacting the components in the presence of a proton acceptor. Typically, the proton acceptors are chosen from bases such as lithium salts, alkali metals, alkali and alkaline earth hydroxides, alkaline earth carbonates and tertiary amines. In addition, the reaction is generally carried out in non-polar solvents such as benzene, toluene and xylene or, alternatively without solvent. While such techniques are adequate to react the components, the processes suffer from slow reaction rates and, consequently produce by-products. In particular, the by-product disulfide is formed from the mercaptophenol due to the presence of traces of oxygen. Further, the predominant use of inorganic proton acceptors results in formation of salts which are not easily removed from the final product.
Japanese Patent Application No. 47-115575 discloses the use of non-polar solvents and polar solvents such as acetonitrile, dioxane, and pyridine; however, it requires the use of a suitable base such as alkali metal, alkali hydroxides and ammonium salts. The reaction times varied from 6 to 8 hours and the final product was recrystallized with n-hexane in order to purify the product.
Due to the difficulties and costs caused by slow reaction times and removal of by-products, it would be desirable to improve the process for preparation of mercaptophenol derivatives. Surprisingly, it has been discovered that by employing an amine base catalyst in a polar solvent the reaction between mercaptophenols and vinyl compounds containing nitrile, carbonyl, sulphone or ester groups proceeds rapidly to form the desired product with very little disulfide formation and; therefore, the product is of high purity. Further, the catalyst can be easily removed by co-distillation with the solvent.